Height maps with large Z offsets

A few users have reported getting G29 height maps that show a large overall Z offset. Until today, the only cause of this that I was aware of was using an endstop switch to do Z homing, where the Z height after homing wasn't quite what the firmware was told in the homing file or the M208 settings.

Today I had an instance of a Z-displaced height map on one of my printers that uses the Z probe for homing, after I temporarily changed the bed surface to one that gave a different trigger height. The error persisted after multiple G29 attempts.

The way I resolved it was:

send G92 S2 G29 S2 to clear the height map

home Z

run G29. This generated a sensible height map.

Sending G92 S2 G29 S2 before homing Z was critical to avoid perpetuating the error in the new height map. I can see now that when the printer is Z-homed using the Z probe, the height map should be re-normalised to the new Z probe trigger point. I will implement this in the next firmware version.

I feel like I might have ran into a similar issue. My printer uses bltouch and I use it for homing and mesh leveling. My heightmap was giving me a ~0.7mm difference yet I’m using a perfectly flat mic6 aluminum plate.

If I probe at 10,10 then lift nozzle 5mm and move to 10,280 (where it’s supposed to be 0.7mm lower) And lower the nozzle at 0, i get the same distance from the bed at both point.